Hemming machines are utilized to reentrantly fold an edge portion of a primary panel against itself or against a secondary panel thereby to reinforce the primary panel edge and present a smooth finished edge surface. Initially, the edge of the primary panel is folded in the stamping process to an angle of approximately 90.degree. relative to the surface thereof. The folded edge of the panel accommodates superposition of a secondary panel in juxtaposed relation thereto. Thereafter, the upstanding edge flange on the primary panel is reentrantly folded or "hemmed" to finish the edge and to secure the secondary panel, if used.
In order to condition the upstanding flange on a panel for hemming, it is necessary to reduce the angle of the upstanding flange from 90.degree. relative to the panel to an angle short of its final folded position, for example 45.degree., but sufficient to insure proper folding of the flange when impacted by the hemming die. This is known in the art as "prehemming".
To accomplish the aforesaid prehemming, prior art hemming operations required two machines, one to effect a first or "prehemming" fold and a second machine to accomplish the final hem. Transfer of the workpiece between two work stations was required. Moreover, hemming machines heretofore known and used utilized a plurality of hydraulic cylinders to effect the required opening, closing and lateral movements of the hemming die. Use of hydraulic cylinders necessitated the use of hydraulic pumps which are characterized by excessive noise and fluid leakage. Moreover, control of the hydraulic cylinders requires relatively complex, inflexible valving.